Fellow f.lux user here. Great app. Best I know w/o 3rd party software or using the monitor's buttons would be to go System Preferences > Displays > Color > Calibrate. Use "Expert Mode". However this will only let you take it down to 4500 K. Interested to find out the answer on this one.
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boehjApr 23 '11 at 20:23

Maybe I don't understand the difficulty, but can't you just set F.lux's daytime slider to 2700? When you say "sometimes", do you mean algorithmically predictable times/dates? If so, then a script would solve your problem. I think it's possible to write a script to force F.lux to a certain daytime temp (or nighttime for that matter).
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KelvinMay 18 '12 at 4:10

Now you can run flux-day-color 2700 in Terminal (or in another script) to change the day temperature. Note that the script restarts F.lux so you may see the display jump to 6500 K for a split second before applying your requested temperature.

It's also possible to schedule this to run at predefined intervals, but that's beyond the scope of this answer (and the question).

If you'd rather have a launchable app that can toggle between 2 temperatures,

Had to check this as an accepted answer as I haven't got Nocturne to work on Mavericks. I enhanced your flux-day-color script a bit and created a Gist that can be used to set either/both day and night temperatures (and just using flux-temp 3000, i.e. without options, to change the current display temperature to whatever regardless of the time.) I also found out the minimum Flux temperature is 2300 K, at least in the current version.
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koiyuNov 10 '13 at 13:54

When using the Displays section of your System Preferences, if you calibrate it, select show advanced options, one of the windows is this:
which to me looks like the white points you were looking for, be sure to uncheck "Use native white point" if you want to manually edit it.

Great info, thanks. I hadn't had the guts to go through all the steps just for the white point. Sadly, though, the lowest kelvin is 4500-4700 (just closed the window ><). Is there a way to adjust just the white point? (well, I could calibrate different profiles) And how to adjust it below the Calibrator's options?
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koiyuApr 23 '11 at 20:31

1

I found the value hardcoded in /Users/username/Library/Preferences/com.apple.ColorSyncCalibrator.plist, but editing it to a lower value had no effect. I will keep looking.
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hobsApr 23 '11 at 20:37

It did have an affect when I logged in and out, I was able to black out my entire monitor, so probably relauching something will reset the value
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hobsApr 23 '11 at 20:41

I think I am going to need to ask my own question, something along the lines of what actions are the sys prefs taking after loading that file to tell the mac to activate the change besides changing the file.
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hobsApr 24 '11 at 0:22

It has a nice amount of options and is completely manual. Downside is it doesn't set apparent color temperature per se, but setting a white tint to RGB(255, 197, 143) has approximately the same effect as setting white point to ≈ 2600 °K (see Kelvin ⇔ RGB table on planetpixelemporium).

Open system preferences, go to the Displays panel and choose the Color tab. Press the Calibrate... button and go through the steps. If you turn on expert mode, you will choose the white point using a slider, but if you don't you will get to choose between a few standard values. You can even do this multiple times, saving with a different name each time, and switch between them through the list in the color tab.